A Dynamic Planet PDF

1. A Dynamic Planet 1.1 The Origins of Geology as a Science The word 'geology' is derived from the Greek word for the earth, 'ge', and 'logos' which means 'to speak'. People really began to take an interest and discuss the origin of the Earth and how the planet around them ‘worked’ during the 1700...

1. A Dynamic Planet 1.1 The Origins of Geology as a Science The word 'geology' is derived from the Greek word for the earth, 'ge', and 'logos' which means 'to speak'. People really began to take an interest and discuss the origin of the Earth and how the planet around them ‘worked’ during the 1700’s, and prior to this, it all seemed a bit mysterious. By the late eighteenth century, the widely accepted theory for the origin and evolution of the Earth was that proposed by Abraham Werner (1749-1817) of Freiburg, in Germany. According to Werner, all rocks had been deposited or crystallized in a few thousand years from an ancient, receding ocean that had originally covered the entire globe. This cleverly kept his geological theory onside with the Church and the Biblical idea of a Noah's Flood. Werner hypothesised that all rocks had been laid down in a continuous and systematic order, with the oldest, or first to form from the ocean, at the bottom, and Abraham Gottlob the youngest forming on top. This neatly Werner (1749-1817) used the French geologist, Nicolas in about 1780, at the height of his career. Desmarest’s (1725-1815) law of From a portrait by an 'superposition', which is still a unknown artist. fundamental principle during the 1 C. J. NICHOLAS A BEGINNER’S GUIDE TO PLANET EARTH deposition of successive layers of sediment, known as strata or beds; the oldest is always at the bottom and the youngest on top. Since Werner’s hypothesis stacked all the rocks we see around us in the world today into a complex series of higglety-pigglety layers, he is often credited as the founder of stratigraphy; that is, the study of strata. Coal deposits burning underground were hypothesised as the source of heat in erupting volcanoes. Werner's theory soon became christened 'Neptunism', after the Roman god of the sea, Neptune. Werner's theory of Neptunism in four easy steps: 1.2 The Great Neptunist - Plutonist Controversy By the end of the eighteenth century a rival hypothesis for the origin and evolution of the planet had been proposed. James Hutton (1726-1797) was an Edinburgh doctor during the heady 2 INTRODUCTION TO GEOLOGY 1. A DYNAMIC PLANET days of the eighteenth century Scottish 'Enlightenment', where he would rub shoulders with the great philosophers and scientists of the period. Hutton's Theory of the Earth as it first However, he also ran two appeared in 1785, in four easy steps: farms in Berwickshire on the coast east of Edinburgh. When he looked at the landscape around him, Hutton realised, for the first time, that we live on a dynamic Sedimentary strata build up on sea floor, burying the older layers deeper and and ever-changing planet. deeper into the crust. Nothing ever stays the same on the surface upon which we live. Even something so seemingly permanent as the rocks we uplift stand on are being destroyed before our very Strata uplifted and deformed to form new mountains, which then begin to eyes in some places, yet erode away. elsewhere at the same time new rocks are forming. Hutton saw that the mountains around him in uplift his home in Scotland, were Sediment derived from mountain actively being weathered erosion transported and deposited in and eroded away. The the sea. resulting sediment was 4. Steps 1. to 3. continue to recycle carried away to the sea again and again. 3 C. J. NICHOLAS A BEGINNER’S GUIDE TO PLANET EARTH where it would eventually be dumped as sheets or layers of sludge on the seafloor (Step 1). Over time, younger layers would be deposited on top of the older ones underneath, gradually burying them deeper and deeper into the Earth's crust. The sludge would harden into rock and then somehow this rock was pushed and squeezed back up to the surface again to form new mountains (Step 2). These mountains would then be exposed to weathering at the surface, and begin to disintegrate to produce new sediment, which itself would then be transported to the sea and dumped again as strata on the seafloor (Step 3). If this cyclicity didn't happen then Hutton hypothesised that there would be no mountains left, as the whole surface of the Earth would simply have been worn away down to sea level (Step 4). The recycling of the rocks which form the Earth's crust meant that in theory, as Hutton famously wrote later, he could see, 'no vestige of a beginning, and no prospect of an end.' In other words, if we can see mountains at the present day being eroded as part of the current cycle, how many cycles of erosion and new mountain formation have there been before this, and how many to come afterwards? Hutton is thus generally regarded as the discoverer of deep, or geological, time. Hutton published his new ‘Theory of the Earth’ as a brief abstract in 1785, but it attracted a lot of criticism from the Neptunists. It was, after all, just a nice theory and what Hutton needed to do was find hard evidence to back-up his claims. In addition, Hutton could not provide a mechanism by which the rocks buried beneath the seafloor could be magically uplifted to the surface again to form new mountains. So, Hutton set off around Scotland in search of field evidence to support his 4 INTRODUCTION TO GEOLOGY 1. A DYNAMIC PLANET theory. He developed a hunch that the mechanism for uplift might be found in the nature of granite and how it was formed, often at the centre of mountains. Finally, in August, 1787, Hutton and his friend and artist, John Clerk, arrived on the Isle of Arran, which lies in the Firth of Clyde, south-west Scotland. Arran is dominated by a large James Hutton (1726-1797) in the field, 1787; the year Northern Granite, and Hutton he visited Arran. From a realised that this might provide the contemporary cartoon by John Kay. Note the rather key evidence that he was looking for. fashionable field clothing and that a geological Hutton wrote later about his hammer was already expedition to Arran: recognised as a crucial piece of field equipment. ‘In setting out upon that expedition, I had but one object in view; this was the nature of the granite, and the connection of it with the contiguous strata. But upon examining the island, I have found it sufficiently interesting and comprehensive to make it the subject of a natural history…. Let us therefore, see how far the natural history of Arran shall be proper to try the Theory of the Earth.’ Eventually Hutton found two key pieces of evidence on Arran which he thought supported his ‘Theory of the Earth’. One was on the north shore by Lochranza, and the second was in a stream bed in North Glen Sannox, just to the 5 C. J. NICHOLAS A BEGINNER’S GUIDE TO PLANET EARTH south. At Lochranza, Hutton found a locality where two quite clear packages of strata lie with an angular discordance, one upon the other, and both tilted in different directions. To Hutton, this was evidence of two full turns of his rock cycle. His reasoning was as follows. Firstly, the layers at the bottom must originally have been laid down horizontally as sediment on the seabed. These were buried, squeezed and tilted as they were brought back to the surface to form mountains. These new mountains were themselves then eroded down to produce another set of horizontal strata deposited on top of them. But these are also tilted, so the whole lot must have subsequently been buried again, squeezed, tilted and uplifted to form the rocks now exposed at Lochranza. Today, we would call this an unconformity, and the Lochranza unconformity’s claim to fame is that it was the first one ever recognized and described. Hutton’s Unconformity at Lochranza; if the strata are assumed to have been deposited originally as horizontal layers on the seafloor, then they must represent two separate cycles of deposition then deformation during uplift. 6 INTRODUCTION TO GEOLOGY 1. A DYNAMIC PLANET Returning home on the same day from Lochranza, Hutton stopped off in North Glen Sannox. Here in the main stream bed he found field evidence of granite veins, or ‘fingers’ intruding into the surrounding schists (schists are a type of metamorphic rock). Hutton surmised that Hutton’s Granite – Schist contact in North Glen the schists must Sannox. Granite veins can still be seen intruding the surrounding country rock, just as Hutton would have have been their seen it in 1787. originally, and that the granite must have been emplaced in a liquid state, in order to produce all the veining. The schists also displayed localized effects of having been heated and hardened where in contact with the veins; a key piece of evidence which Hutton used to infer that not only was the granite liquid, but that it must have been very hot as well. So, here, finally, was Hutton’s evidence that the Earth was hot inside, and hot enough to melt rocks if they were taken deep enough. He concluded that this internal heat in the planet must bend and twist the rocks back up to form new mountains (the mechanism behind uplift in his original Step 2). So, Hutton managed to discover deep, geological time in the morning, and that there is internal heat within the Earth in the afternoon on 7 C. J. NICHOLAS A BEGINNER’S GUIDE TO PLANET EARTH his way home; not a bad day’s work in the field. Whilst, as we shall see, we would now invoke a different mechanism for mountain building, Hutton was right about the Earth being hot inside and this coupled with his ideas of a dynamic, recycling planet were enough to change the way people thought about Earth for ever. Since heat deep in the planet was so crucial to Hutton's theory it became known as ‘Plutonism’, after the Roman god of the underworld, Pluto. Hutton finally published his ideas in a great two volume work entitled, 'Theory of the Earth, with Proofs and Illustrations', in 1795. Sadly, he died in 1797, whilst volume III lay as a manuscript on his desk and what would have been the final volume IV remained unwritten. However, Hutton had achieved enough for many to consider him the 'father' of modern geology. 1.3 The Rock Cycle What Hutton had effectively described is what we would now refer to as the rock cycle. Rocks exposed on the surface of our planet are subjected to chemical and physical weathering which can dissolve their minerals or fracture them into small pieces. Either way, rocks will gradually disintegrate in situ into pieces of rock or individual mineral grains. These products of weathering, known as sediment, can then be carried away by the energetic agents of erosion such as wind, glaciers and ice sheets, or the flowing water of streams and rivers. The sediment may eventually be transported to the sea by rivers and, as their energy is checked by meeting the sea, they may deposit the sediment in layers, which over time gradually accumulate one on 8 INTRODUCTION TO GEOLOGY 1. A DYNAMIC PLANET top of the other. As the older strata are buried deeper and deeper they are gradually compacted and cemented to form sedimentary rock, and if this burial continues, eventually these sedimentary rocks will begin to be squeezed and heated. Increasing pressure and temperature within the Earth's crust can eventually begin to melt or re-align the minerals in the rock, gradually metamorphosing it into something different. The modern day 'text-book' version of the rock cycle. If burial and heating continues, the new metamorphic rock may itself then begin to melt and if this goes to completion then it will form molten magma. Magma is hot, buoyant and less dense 9 C. J. NICHOLAS A BEGINNER’S GUIDE TO PLANET EARTH than solid rock and so will try to force its way upwards towards the surface again. As it rises it will gradually cool and may crystallize to form an intrusive igneous rock. Over time, the layers of rock above it may be eroded away and eventually expose this igneous rock to weathering, and so completing one full turn of the rock cycle. The concept of this cycle is something of an oversimplification as there are various short-circuits or short-cuts that can occur during it. However, it remains a useful way to link the different categories of rock; igneous, sedimentary and metamorphic, to how and where we might expect them to form in, or on, the Earth's crust. 1.4 A Brief Introduction to your Home Planet In essence, our planet consists of four layers. There is a solid Inner Core, a liquid Simplified interior structure of the Earth. Outer Core, a surrounding Mantle, and then the Crust that we live on, forming the thin outer shell of the ‘egg’. All in all, the Earth has an approximate radius of about 6370 km, with the Mantle forming the thickest, single, internal 10 INTRODUCTION TO GEOLOGY 1. A DYNAMIC PLANET layer. Precisely how and why we think there is such an internal structure to the Earth are questions that we will specifically deal with in the next few Chapters. The surface of our Differentiation between Continental Crust, planet is dominated by Oceanic Crust, Crust vs. Mantle, and what constitutes a Lithospheric Plate. the physical features produced by a geological process known as plate tectonics. Evidence hinting at the existence of this process has been observed by geologists for over a century, but it took a more systematic exploration of the ocean floor between the 1950's and 1970's to finally fit all the pieces in place. Because it has become one of the central pillars within geology and critical in understanding how our planet works, we will discuss the origin and evidence behind plate tectonics in some detail over the coming Chapters as well. However, in order to set the scene, we now know that the outermost rigid layer of the Earth is actually divided up into a jigsaw puzzle of variable-sized giant slabs, or 'plates' which move sideways relative to each other over the underlying part of the Earth's mantle. Where these plates move apart, new crust is created and where they collide, plates can be destroyed or form mountains. In fact the 11 C. J. NICHOLAS A BEGINNER’S GUIDE TO PLANET EARTH word tectonics comes from the Greek, '', which roughly translated means 'building'. The theory of plate tectonics has become so widely accepted now that it is sometimes difficult to remember that it is still a theory and needs supporting evidence. If these great Plate movement away from Mid Ocean Ridge slabs, or plates, spreading centres (white arrows) driven by solid-state convection in the Mantle (red arrows). are moving, how do they do it? The answer lies in the somewhat curious physical properties of the mantle below, which although solid, can be made to flow and convect over geological time. The evidence for all of which clearly needs to be teasled-out over the next few Chapters. *** 12